This invention relates to a disc memory apparatus such as disc-shaped recording medium applicable to magnetic disc, and more particularly to an improvement of servo signal recorded at servo zone instituted in detecting positions.
As one type of magnetic discs, there is known a magnetic disc on which servo sectors pre-recorded with information for detecting positions at every predetermined length of tracks such as every sector are formed. Servo sectors of a conventional magnetic disc are shown in FIG. 1. A chaine line indicates a track center of each track of N-1th to N+3th tracks from the outside of the disc on which there are formed the tracks with track pitch Tp.
At the top of the servo sector are recorded a reference signal E which includes an identification signal and so on representing a start position of the servo sector. After the reference signal E are recorded address data with grey code, and subsequently predetermined burst signals are recorded in diced layout.
The burst signals are arranged as follows: first burst signals A1 are recorded on odd-numbered record tracks, and then, second burst signals B1 are recorded on even-numbered tracks. Third burst signals C1 are recorded on locations each of which is distanced by half the track pitch from the first burst signals A1, and fourth burst signals D1 are recorded on locations each of which is also distanced by half the track pitch from the second burst signals B1.
Such an arrangement allows the magnetic disc unit to detect the position of the magnetic head by means of an address data AD when the magnetic head passes along the proximity of a track center TC. The position data detected as above is used to move the magnetic head to a target track.
When the magnetic head passes near the spacing between adjacent record tracks, the magnetic disc unit determines whether the record track which the magnetic head now passes is an odd-numbered track or an even-numbered track, by comparing the signal level of the first burst signal A1 with that of the second burst signal B1.
Furthermore, when the magnetic head passes the Nth record track, for example, the magnetic disc unit can determine whether the magnetic head now passes the inner side or the outer side of the N-th track, by comparing the signal level of the third burst signal C1 with that of the fourth burst signal D1.
When no correct address data AD can be read with the magnetic head passing near the spacing between adjacent record tracks, the above-mentioned structure allows the magnetic disc unit to exactly locate the magnetic head and to perform the seeking operation fast, by using the first burst signals A1 through the fourth burst signals D1 as reference signals.
Furthermore, when the travel speed of the magnetic head is slowed down with a target track being near, the detected address data AD along with the result of comparison of the level of the first and second burst signals A1 and B1 is used, allowing the magnetic disc unit to locate the magnetic head accurately to within an accuracy of the pitch or better.
In the manner mentioned above, the magnetic disc unit achieves disc head placement within a short period of time, thereby resulting a short seek time.
However, the servo sectors, formed as mentioned above, are unavoidably elongated. In seek action, when the servo sector is long and, more particularly, the magnetic head is moved with a high travel speed, the problem is induced in that correct addresses can not read from the servo sector, because the magnetic head is moved across a plurality of tracks.